There is a need to provide calibration equipment of black body sources in the infrared band from 2.5 to 5 micrometers and for calibrating collimators used in testing infrared detectors, optical systems and receivers in the 2.5-5 micrometer band. To provide such calibration, it is necessary to collect and accurately measure 2.5 to 5 micrometer infrared radiation incident on a detector aperture.
Previously one of three different techniques or structures were used for these calibration purposes, viz.: (1) electrically calibrated pyroelectric radiometers, (2) cryogenically cooled quantum detectors, and (3) performing calculations based on the physics of hot sources. Each of these prior art devices or procedures has a number of disadvantages.
Electrically calibrated pyrolytic radiometers generally have relatively low sensitivity, preventing these instruments from providing accurate results below about 1.0 microwatt of incident power. To calibrate sources used in infrared detector testing, accurate measurements must be performed at incident power levels between 1 and 100 picowatt.
Conventional cryogenically cooled infrared quantum detectors employing photovoltaic generators, usually fabricated of indium antimonide (InSb), must be operated in a vacuum dewar enclosure with an infrared transparent window. The InSb element is usually positioned on a cold finger in the vacuum dewar so it can be operated at liquid nitrogen temperatures. Inaccuracies occur because of external reflections, spurious responses due to internal reflection and losses due to reflection from the window. Reflection losses can be reduced by the use of anti-reflecting coatings on the window, but only at the expense of introducing non-uniformities in the spectral transmittance of the window. Typically the anti-reflecting coatings have different transmittivity properties over the 2.5-5 micrometer band of the infrared spectrum being tested.
Calculations based on the physics of infrared sources are based on measurements or assumptions of many parameters including source temperature and emissivity, as well as uniformity in aperture size and distance from the aperture to the source, in addition to spectral transmittance of filters and the atmosphere through which a beam from the source is transmitted. In actuality, these measurements and/or assumptions are frequently not accurate.
It is, therefore, an object of the present invention to provide a new and improved radiometric standard infrared detector.
Another object of the invention is to provide a new and improved radiometric standard infrared detector that collects and accurately measures infrared radiation incident on a detector aperture in the band from 2.5 to 5 micrometers.
A further object of the invention is to provide a new and improved radiometric standard infrared detector for calibration of black body sources and collimators used in testing infrared detectors, infrared optical systems and infrared receivers.
A further object of the invention is to provide a new and improved infrared radiometric standard detector having a linear response over the wavelength band from 2.5 to 5 micrometers and which is accurate for incident power levels as low as 1 picowatt.
A further object of the invention is to provide a new and improved cryogenically cooled infrared quantum detector employing a photovoltaic generator, wherein the detector does not suffer from inaccuracies produced by reflections from a window of a dewar in which the generator is located.
A further object of the invention is to provide a new and improved radiometric standard infrared detector employing a photovoltaic generator in a cryogenically cooled dewar wherein the need for anti-reflecting coatings on a window of the dewar is substantially eliminated.
A further object of the invention is to provide a new and improved radiometric standard infrared detector that does not require calculations based on measurement or assumptions of source temperature, source emissivity, source uniformity, distance between the source and an aperture of the detector or spectral transmittance of filters and the atmosphere.